1. Field of the Invention
The present invention relates to a fillet weld joint and a method for gas shielded arc welding which are of low cost and capable of improving the fatigue strength without involving the risk of causing weld cracking in fillet welding of carbon steel.
2. Description of the Related Art
In recent years, in transportation apparatuses such as automobiles, ships and the like, high tensile strength steel sheets have been more actively used in order to reduce the mass with the aim of improving fuel economy. Similar to mild steel, the high tensile strength steel sheet is arc-welded, however there is a problem that only the fatigue strength similar to that of mild steel can be secured in the weld joint, the fatigue strength of the weld bead lowers than that of the base metal, and intrinsic performance of the high tensile strength steel sheet cannot be exerted.
The main causes the fatigue strength of the weld bead becomes lower than that of the base metal are considered to be (1) the stress concentration at a toe part and (2) the tensile residual stress caused by thermal expansion and shrinkage, and improvements have been tried until now by a number of measures.
With regards to them, relaxing of the stress concentration and lowering the tensile residual stress will be described below.
<Relaxing of Stress Concentration>
In order to relax the stress concentration of a toe part of welding, measures of smoothing the bead shape such as reducing the contact angle and increasing the toe radius have been devised. For example, in Japanese Unexamined Patent Application Publication Nos. H6-340947, H8-25080, 2002-361480 and 2002-361481, it is shown that relaxation of the stress concentration can be achieved by limiting the chemical composition of a steel sheet, use of a wire added with a special composition lowering the surface tension, limiting the welding condition such as the voltage, and the like. However, because there have been many restrictions on conditions, these measures have showed poor versatility. Also, until now, actually there is no technology significantly improving the toe shape only by simply performing welding. Furthermore, according to the result of a confirmation experiment carried out by the present inventors, it was clarified that the fatigue strength improving effect was weak only by relaxation of the stress concentration. That is, improvement of the fatigue strength cannot be achieved only by improvement of the stress concentration.
<Lowering of Tensile Residual Stress (Lowering of Yield Stress of Weld Metal; Due to Improvement of Ductility)>
In Japanese Unexamined Patent Application Publication Nos. H7-171679 and H9-227987, an idea of making the weld metal easily deformable in order to lower the residual stress is proposed. However, in Japanese Unexamined Patent Application Publication No. H7-171679, a concrete welding means was not proposed and was not practical, whereas in Japanese Unexamined Patent Application Publication No. H9-227987, because the deoxidizing composition was reduced excessively and the strength was lowered in a welding wire, there were such problems that a blowhole defect was liable to be generated due to insufficient deoxidization, and that the static tensile strength of the joint became insufficient when applied to a high strength steel sheet.
<Lowering of Tensile Residual Stress (by Stress Relief Annealing)>
As a method for removing a residual stress known best from the past, there is stress relief annealing that retains a high temperature in an annealing furnace (PWHT). However, in the industry related to transportation apparatuses in which the steel material handled is mainly of comparatively thin thickness, the manufacturers possessing a facility are quite rare, and even if such facility is introduced, the production efficiency drops significantly which increases the cost.
<Lowering of Tensile Residual Stress (by Peening)>
There are measures generating a compressive stress after welding called shot peening, hammer peening, and ultrasonic peening, which are proposed in Japanese Unexamined Patent Application Publication No. 2004-136312 also. However, in these measures also, introduction of the facility is required, the production efficiency drops significantly, and the cost is increased.
<Lowering of Tensile Residual Stress (by Low Temperature Transformation Welding Material)>
Under the circumstance, as a measure watched recently, a method is watched in which the martensitic transformation point (Ms point) of the weld metal is lowered and a compressive residual stress is imparted or a tensile residual stress is reduced in expansion transformation at a room temperature. For example, in Japanese Unexamined Patent Application Publication No. S54-130451, a method lowering the Ms point by a weld metal containing both of high Cr and high Ni is proposed. Thereafter, a number of proposals according to the method have been propounded. For example, in Japanese Unexamined Patent Application Publication Nos. 2000-288728, 2001-246495, 2002-273599, 2004-98108, 2004-98109, 2004-98113, 2004-98114 and 2005-238305, a high Cr, High Ni, or high Mn-based weld metal or welding wire is stipulated.
In Japanese Patent No. 4173957, a method for lap fillet welding for a thin sheet is proposed in which the Ms point of a low temperature of 200-350° C. is achieved using a low temperature welding wire of high C content of 0.45-0.7 mass %, and the depth of welding penetration is limited to ⅔ or below of the sheet thickness. Also, in Japanese Unexamined Patent Application Publication No. 2004-136313, a method for lap fillet welding for a thin sheet is proposed in which the Ms point of a low temperature of 250-400° C. is achieved using a low temperature welding wire of high C content of 0.35-0.7 mass % in a similar manner, the depth of welding penetration is limited to ½ or below of the sheet thickness, and the toe part of the weld bead is recessed from the surface of the steel sheet.
On the other hand, in Japanese Patent No. 4173999, a method for fillet welding is proposed in which cracking resistance and absorbed energy of the weld metal are improved by setting the Ms point comparatively high at 475-550° C., and making the depth of welding penetration ⅓ or below of the sheet thickness and the constraining degree 4,000 N/mm·mm.
However, in the fillet weld joints and the methods for fillet welding according to the prior arts, there are such problems as described below.
According to the invention described in all of Japanese Unexamined Patent Application Publication Nos. S54-130451, 2000-288728, 2001-246495, 2002-273599, 2004-98108, 2004-98109, 2004-98113, 2004-98114 and 2005-238305, because a large amount of expensive elements are added and in addition, the wire drawability is inferior when a solid wire is used, the weld material becomes expensive. Also, the worst defect in practical use is a problem that the weld bead becomes a high hardness metal accompanying lowering of the Ms point and increase of the risk of causing delayed cracking (that is, hydrogen cracking) is inevitable. Even if cracking is not caused immediately after welding because the sheet thickness is thin and constraint is less, according to the use environment thereafter, encroachment of hydrogen due to corrosion and embrittlement possibly occur, and the risk of delayed cracking remains. In order to avoid it, it is necessary to perform heat treatment of heating the welded body before and after welding to release diffusible hydrogen, however when a number of joints are mass-produced as in the case of the transportation apparatuses, such procedure is not realistic in fact in terms of the cost. Also, in general, as the martensitic structure becomes dominant, metal becomes brittle and is liable to cause the brittle fracture. Further, the Charpy absorbed energy as a general indicator for such characteristic lowers.
According to the invention described in Japanese Patent No. 4173957 and Japanese Unexamined Patent Application Publication No. 2004-136313, because C content is high, delayed cracking is liable to occur as described above, and hot cracking (that is, solidification cracking) is also liable to occur. Further, the Charpy absorbed energy is also low. In addition, it is also difficult to manage the depth of welding penetration and the recess amount of the bead in actual production.
According to the invention described in Japanese Patent No. 4173999, it is also difficult to manage the depth of welding penetration and the constraining degree in actual production, and the fatigue strength improving effect lowers because the Ms point rises.
As described above, the measures depending upon lowering of the tensile residual stress due to low temperature transformation are not realistic.